A catalyst for coupling carbon dioxide and formaldehyde and its preparation method

Abstract

The present invention provides a catalyst for coupling carbon dioxide and formaldehyde and a preparation method thereof, and relates to the technical field of catalytic reduction of carbon dioxide. The preparation method of the catalyst for coupling carbon dioxide and formaldehyde provided by the present invention comprises: calcining thiourea once, then cooling, pickling, separating and washing in sequence, then calcining twice, cooling and grinding to obtain g-C3N4 nanosheets; dissolving the g-C3N4 nanosheets in water to obtain solution A; taking Zn(NO3)2·6H2O, Al(NO3)3·9H2O and Cu(NO3)2·3H2O to form solution B; mixing solutions A and B with an alkaline solution to react, then centrifuging to obtain a solid product, and drying to obtain. The catalyst for coupling carbon dioxide and formaldehyde provided by the present invention is prepared by the preparation method. The method provided by the present invention improves the visible light response range of the catalyst, improves the product selectivity, and the obtained catalyst has excellent performance and high yield, is simple in process, and has cheap and readily available raw materials, which is suitable for industrial promotion and use.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalytic reduction of carbon dioxide, and in particular relates to a catalyst for coupling carbon dioxide and formaldehyde and a preparation method thereof. Background Art

[0002] Carbon dioxide is the main gas that causes the greenhouse effect. Excessive emissions of carbon dioxide have serious harm to the human living environment. Therefore, it is of great significance to study a catalyst for the efficient and pollution-free reduction of carbon dioxide. Among the many methods for reducing carbon dioxide, photoelectrocatalysis combines the advantages of photocatalysis and electrocatalysis. The photoelectrocatalytic reaction conditions are mild. Both sunlight and an external electric field can have an effect on the reduction of carbon dioxide. At the same time, the effect of the external electric field can also achieve efficient separation of photogenerated electrons and holes. It is a very promising green and environmentally friendly method for reducing carbon dioxide to usable fuels and high-value chemical products. However, research on the photoelectrocatalytic reduction of carbon dioxide is still in its early stages, and there are many reaction products and difficulties in separation. Therefore, the development of a photoelectrocatalyst that is efficient, stable, has excellent light absorption properties, fast transfer of photogenerated electrons and high selectivity is the current main research goal.

[0003] In view of this, the present invention is proposed. Summary of the Invention

[0004] The object of the present invention is to provide a catalyst for coupling carbon dioxide and formaldehyde and a preparation method thereof to solve the above problems.

[0005] In order to achieve the above objectives, the present invention adopts the following technical solutions:

[0006] A method for preparing a catalyst for coupling carbon dioxide and formaldehyde, comprising:

[0007] Thiourea is calcined once, then cooled, acid-washed, separated, and washed with water, and then calcined twice and cooled and ground to obtain g-C3N4 nanosheets. The first calcination of thiourea can produce g-C3N4, but the specific surface area is small; acid washing and re-calcination are to increase its surface area.

[0008] Dissolving the g-C3N4 nanosheets in water to obtain solution A;

[0009] Zn(NO3)2·6H2O, Al(NO3)3·9H2O and Cu(NO3)2·3H2O are mixed to form solution B; the mixed solution B is prepared in order to obtain hydrotalcite;

[0010] Solutions A and B are mixed with an alkaline solution to react, followed by centrifugation to obtain a solid product, which is then dried. A and B are mixed with an alkaline solution to prepare a heterojunction. Double hydroxides, or hydrotalcite structures, must be synthesized under alkaline conditions. If the environment is not alkaline, mixed metal oxides or a mixture of metal oxides may result.

[0011] Preferably, the alkaline solution comprises Na2CO3 and / or NaOH;

[0012] Preferably, the drying is carried out in a vacuum drying oven at 60° C. for 10 to 12 hours.

[0013] Optionally, the primary calcination is carried out at a temperature of 400 to 550° C. and for a time of 2 to 4 hours.

[0014] Preferably, the secondary calcination is carried out at a temperature of 400 to 550° C. and for a time of 2 to 4 hours.

[0015] Optionally, the pickling is performed using an HNO3 aqueous solution;

[0016] Preferably, the pH value of the HNO3 aqueous solution is 0.5 to 2. Preferably, the pH value of the HNO3 is 1. Nitric acid with a pH value of 0.5 to 2 is selected to etch g-C3N4 and increase its specific surface area. Nitric acid can be decomposed into gas by heating, without the need for additional separation and removal.

[0017] Optionally, the water washing includes: continuously stirring in a water bath at 70 to 90° C. for 9 to 10 hours, centrifuging, and washing with water until neutral.

[0018] Optionally, the concentration of solution A is 0.01 to 0.04 g / mL; preferably, the concentration of solution A is 0.02 to 0.03 g / mL.

[0019] Optionally, the usage ratio of Zn(NO3)2·6H2O, Al(NO3)3·9H2O and Cu(NO3)2·3H2O is 1:1:0.005 to 1:1:0.02.

[0020] Optionally, the blending includes: adding the alkaline solution and the solution B to the solution A by a double titration method, so that the prepared catalyst elements are mixed more uniformly.

[0021] Optionally, the blending includes: mixing the solutions A and B with the alkaline solution to form solution C and solution D respectively; fully stirring and mixing the solution C and the solution D in an ice water bath, and then standing at a constant temperature for 8 to 10 hours;

[0022] The ratio of the total amount of the alkaline solution to the amount of the solutions A and B is 3:1:6 to 5:1:6.

[0023] Providing an alkaline reaction environment allows the synthesized catalyst to contain rich alkaline functional groups, optimizing the adsorption performance of reactant molecules such as CO2 on the catalyst surface, while promoting the chemical conversion of the reactants. In addition, the alkaline solution has an etching and pore-forming function during the catalyst synthesis process. The synthesized heterojunction has more defect sites, which increases the sites and locations for adsorption and reaction, further enhancing the photoelectrocatalytic activity.

[0024] Optionally, the reaction is carried out under stirring in a water bath at 50-90° C. for 11-12 hours.

[0025] Experimental verification shows that the reaction is more complete under the constant temperature conditions provided by the present invention, and the overall efficiency and effect are better; if the temperature is too low, the reaction efficiency is low, the reaction is incomplete, and the yield is low; if the temperature is too high, the product will decompose to obtain metal oxides.

[0026] In order to improve the visible light response range of the catalyst, accelerate the separation and transfer of photogenerated electron pairs, and improve product selectivity, the present invention creatively provides a method for loading Cu on a composite material of ZnAl-LDH and g-C3N4 to prepare a type II heterojunction catalyst for the photoelectric coupled reduction of carbon dioxide and formaldehyde.

[0027] A type II heterojunction catalyst for the coupled reduction of carbon dioxide and formaldehyde was prepared by loading Cu on a composite material of ZnAl-LDH and g-C3N4. ZnAl-LDH was prepared by co-precipitation of Zn(NO3)2·6H2O and Al(NO3)3·9H2O. g-C3N4 was prepared using thiourea as the raw material, obtained by calcining thiourea and acid washing. The yield was higher, nearly 10 times that of the existing technology, and the g-C3N4 prepared had a higher density. A coupled photoelectrocatalytic reduction catalyst of carbon dioxide and formaldehyde was prepared by co-precipitation of Zn(NO3)2·6H2O, Al(NO3)3·9H2O and Cu(NO3)2·3H2O in an aqueous solution of g-C3N4. By adjusting the amount of Cu(NO3)2·3H2O added, heterojunction catalysts with different ratios were obtained to meet different application requirements.

[0028] A catalyst for coupling carbon dioxide and formaldehyde is prepared by the preparation method.

[0029] The catalyst provided by the present invention is a type II heterojunction photoelectric catalyst, which is used for coupling carbon dioxide and formaldehyde, has high activity, and can be applied in the field of new environmental catalytic materials.

[0030] Beneficial effects of the present invention:

[0031] g-C3N4 is a two-dimensional layered semiconductor material with a high specific area. It is considered to be a good photoelectric catalyst because of its low preparation cost, response to visible light, and good thermal and chemical stability. However, it still has disadvantages such as limited visible light response and fast recombination rate of photogenerated electrons and holes. LDH, also known as binary hydroxide, and g-C3N4 both have layered structures with adjustable structures. The composite construction of g-C3N4 and ZnAl-LDH into a type II heterojunction catalyst can accelerate the migration rate of carriers through a built-in electric field. It is an effective method to improve the efficiency of photogenerated charge separation and provides a new idea for the development of green, efficient and stable new heterojunction photoelectric catalysts. In addition, the process of the present invention is simple, the raw materials are cheap and readily available, and it is suitable for industrial promotion and use.

[0032] The catalyst for coupling carbon dioxide and formaldehyde provided by the present invention has high activity and a wide range of applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0034] Figure 1 HRTEM image of g-C3N4 nanosheets prepared in Example 1;

[0035] Figure 2 HRTEM image of the catalyst Cu ZnAl-LDH / g-C3N4 prepared in Example 1;

[0036] Figure 3 X-ray diffraction patterns of g-C3N4, ZnAl-LDH and Cu ZnAl-LDH / g-C3N4. DETAILED DESCRIPTION

[0037] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0038] Example 1

[0039] The method provided by the present invention is used to prepare a catalyst for coupling carbon dioxide and formaldehyde, and the specific steps are as follows:

[0040] S1: Weigh 20g of thiourea and place it in a crucible, cover it with a lid and place it in a muffle furnace. Heat it to 500℃ at a heating rate of 5℃ / min and hold it for 4h to obtain a yellow sample. Grind it and then add it to 500ml of HNO3 aqueous solution with pH=1. Stir it continuously in an 80℃ water bath for 9h, centrifuge it, wash it with distilled water until it is neutral, heat it to 500℃ at a heating rate of 5℃ / min, calcine it for 2h, and grind it to obtain g-C3N4 nanosheets. Its HRTEM image is as shown below. Figure 1 As shown;

[0041] S2: Weigh 0.8 g of g-C3N4 prepared in step S1 and place it in 50 mL of deionized water to obtain solution A;

[0042] S3: Weigh 0.097 g Cu(NO3)2·3H2O, 2.974 g Zn(NO3)2·6H2O, and 3.75 g Al(NO3)3·9H2O in 50 mL of deionized water to obtain solution B.

[0043] S4: Weigh 5.3 g of Na2CO3 and place it in 100 mL of deionized water to obtain an alkaline solution;

[0044] S5: Solution A was placed in a water bath at 60°C and stirred. At the same time, solution B and alkaline solution were added to solution A by double titration. After the addition was completed, the mixture was kept for 12 hours. The obtained product was then centrifuged and washed until neutral. The product was vacuum dried at 60°C for 10 hours to obtain the target catalyst. The product was recorded as ZnAl-LDH / g-C3N4. Its HRTEM image is shown below. Figure 2 shown.

[0045] Depend on Figure 1 It can be seen that the obtained g-C3N4 is a layered structure. After acid etching, the flat layer is thinner and has a larger specific surface area. Figure 2 It can be seen that g-C3N4 and ZnAl-LDH are successfully composited, and both have a lamellar structure.

[0046] The X-ray diffraction patterns of g-C3N4, ZnAl-LDH and Cu ZnAl-LDH / g-C3N4 are shown in Figure 2. Figure 3 As shown by Figure 3 It can be seen that Cu ZnAl-LDH and g-C3N4 are successfully composited.

[0047] Example 2

[0048] The method provided by the present invention is used to prepare a catalyst for coupling carbon dioxide and formaldehyde, and the specific steps are as follows:

[0049] S1: Weigh 20 g of thiourea into a crucible, cover it with a lid, and place it in a muffle furnace. Heat it to 530°C at a heating rate of 5°C / min and hold it for 4 h to obtain a yellow sample. Grind it and then add it to 500 ml of a pH = 1 HNO3 aqueous solution. After continuous stirring in an 80°C water bath for 9.5 h, centrifuge it, wash it with distilled water until it is neutral, heat it to 500°C at a heating rate of 5°C / min, calcinate it for 2 h, and grind it to obtain g-C3N4 nanosheets.

[0050] S2: Weigh 0.8 g of g-C3N4 prepared in step S1 and place it in 50 mL of deionized water to obtain solution A;

[0051] S3: Weigh 0.048 g Cu(NO3)2·3H2O, 2.974 g Zn(NO3)2·6H2O, and 3.75 g Al(NO3)3·9H2O in 50 mL of deionized water to obtain solution B.

[0052] S4: Weigh 5.3 g of Na2CO3 and place it in 100 mL of deionized water to obtain an alkaline solution;

[0053] S5: Place solution A in a water bath at 60°C and stir. At the same time, add solution B and alkaline solution to solution A by double titration. After the addition is completed, maintain for 11 hours. Then, centrifuge and wash the obtained product until it is neutral, and vacuum dry it at 60°C for 11 hours to obtain the target catalyst.

[0054] Example 3

[0055] The method provided by the present invention is used to prepare a catalyst for coupling carbon dioxide and formaldehyde, and the specific steps are as follows:

[0056] S1: Weigh 20 g of thiourea into a crucible, cover it with a lid, and place it in a muffle furnace. Heat it to 500 °C at a heating rate of 5 °C / min and hold it for 4 h to obtain a yellow sample. Grind it and then add it to 500 ml of a pH = 1 HNO3 aqueous solution. Stir it continuously in an 80 °C water bath for 9 h, centrifuge it, and then wash it with distilled water until it is neutral. Heat it to 500 °C at a heating rate of 5 °C / min, calcine it for 2 h, and grind it to obtain g-C3N4 nanosheets.

[0057] S2: Weigh 0.8 g of g-C3N4 prepared in step S1 and place it in 50 mL of deionized water to obtain solution A;

[0058] S3: Weigh 0.097 g Cu(NO3)2·3H2O, 2.974 g Zn(NO3)2·6H2O, and 3.75 g Al(NO3)3·9H2O in 50 mL of deionized water to obtain solution B.

[0059] S4: Weigh 5.3 g of Na2CO3 and place it in 100 mL of deionized water to obtain an alkaline solution;

[0060] S5: Solutions A and B are mixed with the alkaline solution obtained in S4 to form solution C and solution D, respectively; solution C and solution D are thoroughly stirred and mixed in an ice-water bath, and then allowed to stand at a constant temperature for 10 hours, and then stirred and reacted in a 60°C water bath for 12 hours, and then the obtained product is centrifuged and washed until neutral, and vacuum dried at 60°C for 10 hours to obtain the target catalyst.

[0061] Comparative Example 1

[0062] According to the steps provided by the present invention, the catalyst is prepared without adding Cu(NO3)2·3H2O. The specific steps are as follows:

[0063] S1: Weigh 20 g of thiourea into a crucible, cover it with a lid, and place it in a muffle furnace. Heat it to 550°C at a heating rate of 5°C / min and hold it for 4 h to obtain a yellow sample. Grind it and then add it to 500 ml of a pH = 1 HNO3 aqueous solution. Stir it continuously in an 80°C water bath for 10 h, centrifuge it, and then wash it with distilled water until it is neutral. Heat it to 500°C at a heating rate of 5°C / min and calcine it for 2 h. Grind it to obtain g-C3N4 nanosheets.

[0064] S2: Weigh 0.8 g of g-C3N4 prepared in step S1 and place it in 50 mL of deionized water to obtain solution A;

[0065] S3: Weigh 2.974 g Zn(NO3)2·6H2O and 3.75 g Al(NO3)3·9H2O in 50 mL of deionized water to obtain solution B.

[0066] S4: Weigh 5.3 g of Na2CO3 and place it in 100 mL of deionized water to obtain an alkaline solution;

[0067] S5: Place solution A in a water bath at 60°C and stir. At the same time, add solution B and alkaline solution to solution A by double titration. After the addition is completed, keep it for 11 hours. Then, centrifuge and wash the obtained product until it is neutral. Vacuum dry it at 60°C for 12 hours to obtain the target catalyst, which is recorded as ZnAl-LDH / g-C3N4.

[0068] The performance of the catalysts prepared in Examples 1-3 and Comparative Example 1 was tested, and pure g-C3N4 and pure ZnAl-LDH were selected as comparisons. The test conditions were as follows:

[0069] AM 1.5G, light intensity 100mW / cm 2A high uniformity xenon lamp was used as the light source, and high purity CO2 was introduced into 3M paraformaldehyde and 1M KOH electrolyte for 30 min to saturate the solution with CO2. 2 ) is the counter electrode, the prepared catalyst is the photocathode, and the saturated Ag / AgCl electrode is the reference electrode. Under light conditions, a negative bias of -1.1V (vs.Ag / AgCl) is applied to catalytic reduction of carbon dioxide and formaldehyde for 3h. The obtained reduction products are shown in Table 1 below:

[0070] Table 1 Catalytic performance test results of the catalyst of the present invention for coupling carbon dioxide and formaldehyde

[0071]

[0072] As can be seen from Table 1, compared with pure g-C3N4, pure ZnAl-LDH and the catalyst ZnAl-LDH / g-C3N4 prepared in the absence of Cu in Comparative Example 1, the catalyst Cu ZnAl-LDH / g-C3N4 prepared in Examples 1-3 has a high reactant conversion rate at the same potential and a high selectivity for the target product HCOOCH3. The preparation method provided by the present invention forms a g-C3N4 / ZnAl-LDH heterojunction by combining g-C3N4 and ZnAl-LDH. The internal interface contact of the heterojunction is closer, the spatial separation and transmission efficiency of photogenerated electrons and holes are significantly improved, and the spectral response range is increased. In addition, the high-porosity layered structure of the heterojunction provides more reactive sites and reaction sites for the adsorption and conversion of reactive molecules, thereby improving the catalytic efficiency.

[0073] Please note that the technical features of the above embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification. The above embodiments only express several implementation methods of the present application. The description is relatively specific and detailed, but it cannot be understood as a limitation on the scope of the invention patent. It should be pointed out that for ordinary technicians in this field, without departing from the concept of this application, several variations and improvements can be made, which all fall within the scope of protection of this application. Therefore, the scope of protection of the patent in this application shall be based on the attached claims.

Claims

1. A method for preparing a catalyst for coupling carbon dioxide and formaldehyde, characterized in that: include: The thiourea is calcined once, then cooled, acid-washed, separated and washed with water, and then calcined twice, cooled and ground to obtain g-C3N4 nanosheets; Dissolving the g-C3N4 nanosheets in water to obtain solution A; Take Zn(NO3)2·6H2O, Al(NO3)3·9H2O and Cu(NO3)2·3H2O and mix them to form solution B; The solutions A and B are mixed with the alkaline solution to react, and then centrifuged to obtain a solid product, which is then dried.

2. The preparation method according to claim 1, characterized in that The primary calcination temperature is 400-550° C., and the time is 2-4 hours.

3. The preparation method according to claim 1, characterized in that The pickling is carried out using an HNO3 aqueous solution; The pH value of the HNO3 aqueous solution is 0.5-2.

4. The preparation method according to claim 1, characterized in that The water washing comprises: continuously stirring for 9 to 10 hours in a water bath at 70 to 90° C., centrifuging, and washing with water until the mixture is neutral.

5. The preparation method according to claim 1, characterized in that The concentration of the solution A is 0.01-0.04 g / mL.

6. The preparation method according to claim 1, characterized in that The usage ratio of Zn(NO3)2·6H2O, Al(NO3)3·9H2O and Cu(NO3)2·3H2O is 1:1:0.005 to 1:1:0.

02.

7. The preparation method according to claim 1, characterized in that The blending includes: adding the alkaline solution and the solution B into the solution A by a double titration method.

8. The preparation method according to claim 1, characterized in that The blending comprises: mixing the solutions A and B with the alkaline solution respectively to form solution C and solution D respectively; fully stirring and mixing the solution C and the solution D in an ice water bath, and then standing at a constant temperature for 8 to 10 hours.

9. The preparation method according to any one of claims 1 to 8, characterized in that The reaction conditions are stirring in a water bath at 50-70° C. for 11-12 hours.

10. A catalyst for coupling carbon dioxide and formaldehyde, characterized in that: The invention is prepared by the preparation method according to any one of claims 1 to 9.

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